In recent years, a mobile phone comes into widespread use, and the mobile phone is used in a variety of places. The mobile phone is used not only in quiet places but also in noisy environments such as the airport lobby and the railroad station platform.
Therefore, in order that a listener can easily hear the voice of a speaker in the noisy environment, a method is known, in which a high frequency region of the voice is emphasized depending on the ambient noise level (see, for example, Patent Document 1).
In the case of the method described in Patent Document 1, the voice is output at a level larger than that of the noise in a band to be emphasized. However, if the sound volume exceeds the limit of the output performance of a speaker, then the voice is distorted, and the sound quality is contrarily deteriorated in some cases. In another situation, it is feared that the high level output of the voice may exert any harmful influence on the auditory organ of the listener.
In view of the above, the following method has been suggested. That is, the received voice is recorded in a memory beforehand if the ambient noise level is large. When the ambient noise level is decreased, the simultaneous recording/reproduction (follow-up reproduction) is performed. Accordingly, the received voice can be heard with ease even in the highly noisy environment (see, for example, Patent Document 2).    [Patent Document 1] Japanese Laid-Open Patent Publication No. 11-202896    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2007-312040    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2002-258881    [Patent Document 4] Japanese Laid-Open Patent Publication No. 2007-003682    [Patent Document 5] Japanese Laid-Open Patent Publication No. 2002-287800    [Patent Document 6] Japanese Laid-Open Patent Publication No. 2000-349893    [Patent Document 7] Japanese Laid-Open Patent Publication No. 10-049191